1. Field of the Invention
This application relates to the field of water and waste water treatment. More particularly, this application relates to a membrane system for treating water and waste water.
2. Description of the Related Art
While there are many methods to remove impurities from water, membrane treatment is becoming far more common as technologies improve and water sources become more contaminated. Membrane treatment entails providing a pressure differential across a semi-permeable membrane. The differential allows relatively smaller water molecules to flow across the membrane while relatively larger contaminants remain on the high pressure side. As long as the contaminants are larger than the pores in the membrane, they can be effectively filtered out by the membrane and removed with the concentrate.
Different membranes can be used for different raw water sources and treatment goals. Classifications of membranes generally fall into four broad categories, generally defined by the size of contaminants screened out by the membrane. This size can loosely be correlated to the pore size in the membrane. The four broad categories of membranes are, in decreasing order of the size of materials screened, microfiltration (MF) membranes (which are capable of screening materials with atomic weights between about 80,000 and about 10,000,000 Daltons); ultrafiltration (UF) membranes (which are capable of screening materials with atomic weights between about 5,000 and about 400,000 Daltons); nanofiltration (NF) membranes (which are capable of screening materials with atomic weights between about 180 and about 15,000 Daltons); and reverse osmosis (RO) membranes (which are capable of screening materials with atomic weights between about 30 and about 700 Daltons).
MF and UF membrane systems are typically operated under positive pressures of, for example, 3 to 40 psi, or under negative (vacuum) pressures of, for example, −3 to −12 psi, and can be used to remove particulates and microbes. MF and UF membranes may be referred to as “low-pressure membranes.” NF and RO membranes, in contrast, are typically operated at higher pressures than MF and UF membrane systems, and can be used to remove dissolved solids, including both inorganic and organic compounds, from aqueous solutions. NF and RO membranes may be referred to as “osmotic membranes.” Osmotic membranes are generally charged, adding to their ability to reject contaminants based not only on pore size but also on the repulsion of oppositely-charged contaminants such as many common dissolved solids. Reverse osmosis (RO), nanofiltration (NF) and, to some extent, ultrafiltration (UF) membranes can be used in cross-flow filtration systems which operate in continuous processes (as opposed to batch processes) at less than 100% recovery.
RO and NF membranes can be composed of a thin film of polyamide deposited on sheets of polysulfone substrate. One common form of RO or NF membrane is a thin film composite flat sheet membrane that is wound tightly into a spiral configuration. UF membranes are more commonly provided as hollow fiber membranes, but can also be used in spiral wound elements. The spiral elements make efficient use of the volume in a pressure vessel by tightly fitting a large area of membrane into a small space. A spiral element typically consists of leaves of back to back flat sheet membranes adjoining a perforated tube. Between the back to back membranes of each leaf is a permeate carrier sheet that conveys the treated water around the spiral to the central perforated collection tube. A feed water spacer is wound into the spiral to separate adjacent leaves. After the leaves are wound against each other they are as close together as 0.5 to 0.8 millimeters (about the thickness of the physical feed (raw water) spacer that is rolled up with the membrane leaves). The feed water spacer maintains an adequate channel between the membrane layers so that pressurized feed water can flow between them.
Feed channel spacers typically consist of a netting of cylindrical fibers. These fibers impede the flow of the water down the channel, creating “dead spaces” of little or no water movement both upstream and downstream of the feed spacer fibers. FIG. 7 shows a longitudinal cross section of one such feed channel (taken perpendicular to the direction of winding). The flow direction 29 is indicated by the arrow in FIG. 7. In the dead spaces 30 upstream and downstream of the cylindrical fibers 23, small particles 25 and bacteria settle and accumulate. The dead spots 30 are regions with virtually no flow velocity and represent perfect conditions for particles 25 to settle and for bacteria to colonize. Particle settlement 25 in these dead spots fouls the membrane. Over time, as the particles accumulate and biological organisms reproduce, this fouling can stop the longitudinal flow 29 of water in the inter-membrane spaces and can slow the penetration of permeate through the membrane 24.
Fouling is the single greatest maintenance issue associated with membrane water treatment. Fouling occurs when contaminants in the water adhere to the membrane surfaces and/or lodge into the membrane pores. Fouling creates a pressure loss in the treatment process, increasing energy costs and reducing system capacity. Numerous cleaning methods have been developed to de-foul membranes but they are complex, require significant downtime and often do not fully restore the flux of the membranes.